fix the digital ocean registry example

[exterm]

It didn't work as shown.

Actually I'm a little confused - the same example exists in https://github.com/basecamp/kamal-site. Do I need to fix it there too?

[nickhammond]

@exterm basecamp/kamal-site#155 Does this update work for you?

[exterm]

@nickhammond partially - I do think the registry URL needs to include the name of the registry (registry.digitalocean.com/<your-registry>).

[exterm]

Also - why is the documentation here and there? Where do we need to change it?